The present invention relates to a method and apparatus for converting a feed material including pulverulent sulfidic metals to a cogent agglomerated mass of the corresponding metallic oxides. The agglomerated mass is suitable as the feed material to a blast furnace for the reduction of the metallic oxides and recovery of the elemental metal. Specifically, the invention provides a sintering system for sulfidic metals that operates efficiently without the emission of sulfur containing gases, i.e. sulfur dioxide, to the atmosphere and that recovers for useful purposes a significant portion of the heat generated in burning the sulfidic content of the feed material.
For many decades, metals have been recovered from ore containing the metal in sulfidic form, i.e., as metal sulfates and sulfides, by grinding the ore to pulverulent form, concentrating the ore to remove impurities, sintering the concentrate to form an agglomerated mass of the corresponding metallic oxide and reducing the metallic oxide in a blast furnace to the elemental metal. The sintering step is usually performed on an apparatus known in the art as the Dwight and Lloyd sintering machine.
The Dwight and Lloyd machine provides an endless moving grate similar to a conveyor belt, upon which a pulverulent layer of ore concentrate containing sulfidic metal travels. At one end of the machine there is an ignition chamber in which the sulfidic content of the concentrate is ignited. Air and fuel is blown downwardly through the concentrate to initiate combustion at the lower surface of an ignition layer of concentrate. The main feed layer of concentrate is added on top of the ignited ignition layer and the combination is conveyed by the grate to a sintering zone wherein air is blown upwardly therethrough to support the combustion of the sulfur containing compounds. The gas coming off the upper portion of the sintering zone is rich in sulfur dioxide content and may be supplied to a sulfuric acid plant as a feed material.
The concentrate is burned in the sintering zone until the flame breaks through its upper surface, at which point a maximum temperature is reached and most of the sulfur has been burned off as sulfur dioxide. The burning step agglomerates the concentrate into a cogent mass, generally known in the art as a sinter strand (hereinafter referred to as "strand").
The strand leaving the sintering zone is transported by the grate to a cooling zone located adjacent to and downstream from the sintering zone. A stream of cooling gas is blown through the strand and is collected in a hood disposed above the grate. After cooling, the strand is physically reduced, fines are returned to the ignition chamber and the coarse sinter is fed to a blast furnace for reduction of the metal oxides to elemental metal.
Since some burning of the strand continues downstream of the sintering zone, the exhaust gas from the cooling zone can contain sulfur dioxide. However, the exhaust gas from the cooling zone is too lean in SO.sub.2 content to be supplied to a sulfuric acid plant and has been discharged into the atmosphere or treated to remove SO.sub.2.
Accordingly, it is an objective of the invention to provide a sintering method and apparatus that greatly reduces the emission of SO.sub.2 to the atmosphere and ideally eliminates SO.sub.2 emissions. It is a further objective of the invention to provide a sintering method and apparatus wherein a substantial portion of the heat generated is recovered, resulting in lower fuel consumption for the industrial installation in which it is utilized. Still further, it is an objective of the invention to concentrate substantially all of the evolved SO.sub.2 into a single gas stream for use as feed to a sulfuric acid plant. These and further objectives and advantages of the invention will become apparent to one skilled in this art as the description of the invention proceeds.